Control of induction period in blueprint paper



Fatented July 6, i943 2,323,798 T QFWCE CONTROL OF INDUCTION PERIOD IN BLUE- I PRINT PAPER.

Clyde A. Crowley, Chicago, and George B. Good-- year, Wilmette, lill., assignors to The Huey 00., Chicago, 111., a corporation of Illinois No Drawing. Application June 14, 1941, Serial No. 398,112

8 Claims.

Our invention relates to improvements in the formulation of sensitizers for light-sensitive paperscommonly known as blueprint papers, and the control of the light-reacting period of these sensitizers. This application is a continuation in part of our copending application, Serial No. 331,391, in which the control of the light-reacting period of blueprint papers has been discussed in considerable detail, and in which specific means for controlling the light reaction have been disclosed.

For many years sensitizers have been formulated using: (1) a light sensitive'ferric salt such as ferric ammonium citrate or ammonium ferrioxalate; (2) a ferricyanide salt; and (3). other salts such as potassium oxalate, citrates, etc., which help keeping and washing properties, but do not themselves enter into the composition of the final pigment. In fact, their use in any amount more or less decreases the intensity and quality of the final blue pigment. When too much is used, the color becomes unacceptable. Also, their presence markedly decreases the sensitivity of the paper to light.

In practice, the light sensitive ferric compound has been used in great excess over the quantity which is chemically equivalent to the ferricyanide salt (customarily potassium ferricyanide). This is necessary to obtain sufilcient light sensitivity.

Ferricyanide is red in color and seems to filter out the shorter wave lengths of light which are most effective in causing the photo-chemical reaction. Therefore, to obtain high printing speed papers with this type of formulation, the stoichiometric ratio of potassium ferricyanide to reducible ferric iron must be very small. As ferricyanide is one of the essential components required to produce the final pigment, the maximum possible quantity of pigment, and hence the intensity of color possible, is limited by this component. If even slightly over-exposed, ferrous iron in excess over the ferricyanide is formed and this ferrous iron in the development process washes over into the unexposed portions, where it reacts with ferricyanide to form blue pigment. This is called bleeding.

Also, the presence of excess ferrous iron in the exposed portions adversely affects the quality of the blue pigment, giving it an undesirable slate or greyish cast. This is called fburni-ng out."

If, in the sensitizer, the ratio of potassium ferricyanide to light sensitive ferric compound is increased, the speed is decreased but the other desirable properties are gained in proportion. The sensitizers used by commercial blue print paper manufacturers represents an unsatisfactory compromise of these inherent characteristics.

When a paper is exposed,'the ferric compound must be reduced to a ferrous compound and the ferrlcyanide salt to a ferrocyanide salt. These react to form insoluble ferro ferrocyanide. Development consists of washing of! the unexposed and excess chemicals, followed by the oxidation of the ferro-ferrocyanide to Prussian blue, and then washing off the excess oxidizing agent, which is usually potassium bichromate but may be any other suitable oxidizing agent.

Ferrocyanide salts have heretofore been suggested in sensitizer compositions. British Patent No. 418,369 to the Halden Co., and U, S, Patents No. 2,093,738 and No. 2,126,504 to Reynolds have disclosed formulas and limited rules for formulation. When ferrocyanide was added to the customary formulations, the following observations were made.

The speed of the paper was increased; however. in order to make it possible to wash the unexposed areas clean, it was necessary to add much more of the solubilizing salts such as oxalates, citrates. and phosphates, and to use a substantial proportion of a ferricyanide salt. If the proportion of ferricyanide was decreased much below one-half the total quantity of iron cyanides, it was then necessary to add such a large proportion of solubilizing salts that the quality of the final blue pigment was seriously impaired and the speed was reduced. When ferrocyanides were used in equal amount with the ferricyanide, the necessary amount of solubilizing salts markedly desensitized the paper. The result was that it seemed impossible to add more than a certain limited amount of ferrocyanide because the addition of greater quantities required additional ferricy anide, which made the paper so slow it was not acceptable in commercial practice. If large quantities of solubilizing salts were used instead of ferricyanide, the quality of color was greatly impaired. The total amount of ferrocyanide and ferricyanide in formulas so far disclosed is far below that theoretically equivalent to the light sensitive ferric compound present. The formulas disclosed which have the higher proportion of total iron cyanides are very slow from the standpoint of printing speed.

Some years ago we noted a phenomenon in the exposure of blueprint papers which we chose to call the induction period." This can best be explained as follows:

If paper is coated with a sensitizer consisting of a light sensitive iron complex and a ferricyasame test repeated, it will be found that no color develops on the shortest exposure, but that when the color does start appearing. it does so almost as rapidly as in the initial test. If, now, the quantity, of potassium oxalate is doubled, the period of exposure required to give the first trace of color is also increased to approximately twice as long.

We have named this initial period of exposure which results in no formation of pigment, the induction period. This induction period is a very important characteristic of blueprint papers. It enables them to be used in well lighted rooms as long as there is no direct sunlight, and assists in giving better lines from old tracing where some of the lines may be worn thin. It promotes the contrast of the paper.

We have found that papers coated with sensitizers containing a light sensitive ferric compound, aferrocyanide salt, and a solubilizing salt, have noor at most a negligible induction period. In our copending application, Serial No. 331,391, we have disclosed our discovery that certain classes of oxidizing agents when added in small quantities to a blueprint sensitizer containing ferrocyanides, control the induction period of this class of sensitizers (containing ferrocyanides), just as does a solubilizing salt in the previous type of sensitizer (containing ferricyanides) In said application, we disclosed the fact that not all oxidizing agents are capable of thus controlling the induction period; and have described those oxidizing agents which can control the induction period of this class of blueprint sensitizers as light-reaction-control agents. Our application Serial No. 331,391, discloses, among other things, a. class of light-reactioncontrol agents comprising oxidizing agents characterized by reducible anion containing a metal, which oxidize by virtue of a valance change in said metal.

Our present invention is concerned with the utilization of another class of oxidizing agents which are capable of functioning as light-reaction-control agents to fix the induction period at a desirable value. This class of oxidizing agents is characterized by a peroxy group, oxidation occurring by virtue of oxygen liberated in the transformation of such compounds to normal salts of oxides. Examples of this class of oxidizing agents are peroxides, such as hydrogen peroxide, and persulphates, such as potassium persulphate and ammonium persulphate.

Typical reactions of compounds containing a peroxy group, illustrating the liberation of oxygen, are the following:

Oxidizing agents characterized by a peroxy group, such as peroxides and per-salts in general, are found to function as light-reactioncontrol agents in establishing the induction period at a desired value, in a manner equivalent to the similar action as light-reaction-control agents of oxidizing agents characterized by a reducible anion containing a metal. It is found that the effectiveness of oxidizing agents of either class as light-reaction-control agents is approximately the same on a basisof chemical equivalents, i. e., a mol of potassium ferricyanide gains one electron when it is reduced; 8. mol of hydrogen peroxide gains two electrons; and a mol of potassium persulphate also gains two electrons.

The phenomenon of the induction period has very frequently been confused with the speed of a paper. Actually, it should be considered as only one factor in the performance of the paper. Other factors have considerable influence on the actual rate of the photochemical reaction; for example, the proportion of light sensitive ferric compound present. The larger the proportion of this compound or compounds in the sensitizer, the more sensitive will paper coated with this sensitizer be.

It has been found that the pH of these sensitizing solutions is not critical between reasonable limits. The pH .must not be so high as to precipitate the iron from the light sensitive ferric compound as ferric hydroxide or basic ferric compounds. This pH varies with the nature and quantity of the solubilizing salts present, being in general lower in the presence of phosphates and higher in the presence of citrates. A pH of 8.5 or 9.0 is approximately the upper limit.

When the pH is much below 4.0, it becomes more difficult to stabilize the paper from the standpoint of shelf life; however, we do not wish to limit the practice of this invention by any minimum pH value as certain classes of blue print papers falling within the definitions of this invention can be manufactured from sensitizers adjusted to pH values below a pH of 4.

Further evidence of the non-critical pH range is indicated by the following observations. The sensitizing coating has been washed, with distilled water, from satisfactory papers containing ferrocyanide salts. The pH of these washings invariably has been found to be between 5.5 and 6.9. This was also found to be true for experimental papers made from formulas disclosed in the previously referred to patents issued to Reynolds and to the Halden Co. In general, we have observed that regardless of the pH of the original sensitizing solution, the pH of washings removed from the sensitized sheet invariably falls between 5.5 and 6.9.

Keeping qualities of the sensitized paper were found to depend more upon the particular positive ions present than upon any other factor. When sensitizers contain only ammonium ferrioxalate as the light sensitive ferric compound, papers so sensitized have poor stability. Using the same formula, but replacing the ammonium ferrioxalate with an equivalent quantity of sodium ferrioxalate resulted in a sensitizer which when coated on a paper resulted in a remarkably stable product. However, the quality of the blue background of prints was not as good. In practice, mixtures of the two salts seem desirable. It seems that the amount of water of hydration of the salts has much to do with the stability of the sensitized paper.

Examples of solutions of the type disclosed follow.

The induction period of the above is controlled by the addition of small amounts of hydrogen peroxide, potassium persulphate, or other oxidizing agents containing a peroxy group. For most applications, the addition of 0.06 to 0.31 g. of D0- tassium persulphate gives the proper control. It will be seen that 0.06 to 0.31 g. of potassium persulphate comprises 1.2% to 6.2% of the amount of potassium ferrocyanide employed in the above Example No. I. If no light-reactioncontrol agent is added, the sensitized .paper is not satisfactory for most applications.

Example No. II

The induction period of the above may be controlled by the addition of small amounts of potassium persulphate or other oxidizing agents containing a peroxy group. For most applications, the addition of 0.08 to 0.37 g. of potassium persulphate gives the proper induction period. This amount of potassium persulphate represents 0.8% to 3.7% of the amount of potassium ferrocyanide used in Example No. II.

While the substances which we have found to function as light-reaction-control agents are oxidizing agents of the particular type described, it will be understood that the actual mechanism by which this end is accomplished is subject to uncertainties which cannot be defined further in the present state of the art; and no unnecessary limitation of our invention is to be assumed because we have thus described their nature.

We claim:

1. A sensitizer for blueprint paper and the like comprising a light-sensitive ferric complex, a ferrocyanide salt in amounts such that substantially all of the pigment formed results from the ferrocyanide present in the sensitizer, solubilizing salts, and an oxidizing agent characterized by a. peroxy group in a quantity sufiicient to function as a light-reaction-control agent to establish a predetermined induction period, said quantity of oxidizing agent, when calculated to an equivalent amount of potassium persulphate, not exceeding a few percent by weight of the amount of ferrocyanide salt present, calculated as potassium ferrocyanide, and said solubilizing salts being present in amounts materially greater than the amount of the oxidizing agent when calculated to an equivalent amount of potassium persulphate.

2. A sensitizer for blue print paper and the like comprising a light-sensitive ferric complex selected from the group consisting of ferric ammonium citrate and ferrioxalates, a ferrocyanide salt in amounts such that substantially all of the pigment formed results from the ferrocyanide present in the sensitizer, solubilizing salts, and an oxidizing agent characterized by a peroxy group in a quantity sufllcient to function as a light-reaction-control,agent to establish a predetermined induction period, said quantity of oxidizing agent, when calculated to an equivalent amount of potaissum persulphate, not exceeding a few percent by weight of the amount of ferrocyanide salt present, calculated as potassium ferrocyanide, and said solubilizing salts being present in amounts materially greater than the amount of the oxidizing agent when calculated to an equivalent amount of potassium persulphate.

3. A sensitizer for blue print paper and the like comprising a light-sensitive ferric complex, po-

tassium iferrocyanide, solubilizing salts, and a peroxide in a quantity suflicient to act as a lightreaction-control agent to establish a predetermined induct on period, said quantity of peroxide comprising no in excess of about 1% by veight of the amoun of potassium ferrocyanide, the solubilizing salts being present in amounts materially greater than the amount of the peroxide.

4. A sensitizer for blue print paper and the like comprising a light-sensitiv ferric complex selected from the group consisting of ferric ammonium citrate and ferri-oxalates, a ferrocyanide salt in amounts such that substantially all of the pigment formed results from the ferrocyanide present in the sensitizer, solubilizing salts, and an oxidizing agent characterized by a peroxy group in a quantity sumcient to function as a light-reaction-control agent to establish a predetermined induction period, said quantity of oxidizing agent, when calculated to an equivalent amount of potassium persulphate, not exceeding about 8% by weight of the amount offerrocyanide salt present, calculated as potassium ferrocyanide, and said solubilizing salts being present in amounts materially greater than the amount of the oxidizing agent when calculated to an equivalent amount of potassium persulphate.

5. .A light sensitive paper having a predetermined induction period, said paper having a coating thereon of a sensitizer comprising a lightsensitive ferric complex, a ferrocyanide salt in amounts such that substantially all of the pigment formed results from the ferrocyanide present in the sensitizer, solubilizlng salts, and an oxidizing agent characterized by a peroxy group in a quantity suflicient to function as a lightreaction-control agent to establish a predetermined induction period, said quantity of oxidizing agent, when calculated to an equivalentamount of potassium persulphate, not exceeding a few percent by weight of the amount of ferrocyandie salt present, calculated as potassium ferrocyanide, and said solubilizing salts being present in amounts materially greater than the amount of the oxidizing agent when calculated to an equivalent amount of potassium persulphate.

6. A light sensitive paper having a predetermined induction period, said paper having a coating thereon of a sensitizer comprising a lightsensitive ferric complex selected from the group consisting of ferric ammonium citrate and ferrioxalates, a ferrocyanide salt in amounts such that substantially all of the pigment formed results from the ferrocyanide present in the sensitizer, solubilizing salts, and an oxidizing agent characterized by a peroxy group in a quantity suflicie'nt to function as a light-reaction-control agent to establish a predetermined induction period, said quantity of oxidizing agent, when calculated to an equivalent amount of potassium persulphate, not exceeding a few percent by weight of the amount of ferrocyanide salt present, calculated as potassium ferrocyanide, and said solubilizing salts being present in amounts materially greater than the amount of the oxidizing agent when calculated to an equivalent amount of potassium persulphate.

7. A light sensitive paper having a predetermined induction period, said paper having a coating thereon of a sensitizer comprising a lightsensitive ferric complex selected from the group consisting of ferric ammonium citrate and ferrioxalates, a ferrocyanide salt in amounts such that substantially all of the pigment formed results from the ferrocyanide present in the sensitizer,

mined induction period, said paper having a coating thereon of a sensitizer comprising a lightsensitive ferric complex, potassium terrocyanide, soiubilizing salts, and hydrogen peroxide in a quantity sufilcient to act as a light-reaction-control agent to establish a predetermined induction period, said quantity of hydrogen peroxide comprising not in excess of about 1% by weight of the amount of potassium ferrocyanide, the solubilizing salts being present in amounts materially greater than the amount of the hydrogen peroxide.

CLYDE A. CROWLEY.

GEORGE H. GOODYEAR. 

